Tafseer Ibn Kathir of THAAHAA [76]

جَنَّاتُ عَدْنٍ تَجْرِي مِن تَحْتِهَا الْأَنْهَارُ خَالِدِينَ فِيهَا وَذَلِكَ جَزَاء مَن تَزَكَّى

Gardens of perpetual residence beneath which rivers flow, wherein they abide eternally. And that is the reward of one who purifies himself.